Current collector assemblies for electrochemical cells have been used for many years. Current collectors for alkaline manganese dioxide electrochemical cells are well known. Numerous disclosures of such current collector assemblies have been made, including the following patents, all of which are hereby incorporated by reference in their respective entireties.
______________________________________ Country Patent No. Inventor Issue Date ______________________________________ U.S.A. 3,042,734 Carmichael et al. 1960 U.S.A. 3,314,824 Spanur 1967 U.S.A. 3,617,386 Bosben et al. 1971 U.S.A. 3,663,301 Ralston et al. 1972 U.S.A. 3,740,271 Jammet et al. 1973 U.S.A. 3,764,392 Kuwazaki et al. 1969 U.S.A. 3,954,505 Anderson 1976 U.S.A. 4,075,398 Levy 1978 U.S.A. 4,191,806 Levy 1980 U.S.A. 4,227,701 Tsuchida et al. 1980 U.S.A. 4,476,200 Markin et al. 1984 U.S.A. 4,537,841 Wiacek et al 1985 U.S.A. 4,670,362 Wiacek et al. 1987 U.S.A. 5,008,161 Johnston 1991 U.S.A. 5,015,542 Chaney et al. 1991 U.S.A. 5,051,323 Murphy et al. 1991 U.S.A. 5,227,261 Georgopoulos 1993 U.S.A. 5,248,568 Getz 1993 U.S.A. 5,422,201 Georgopoulos 1995 ______________________________________
U.S. patent application Ser. No. 08/407,391, filed Mar. 20, 1995 and entitled "Battery Sealing Cap," is also hereby incorporated by reference in its entirety.
The disclosure herein discusses the invention within the context of a cylindrical alkaline manganese dioxide cell. However, the invention is not limited to alkaline manganese dioxide cells. Rather, the invention can be practiced with a wide variety of cell structures, and incorporating a wide variety of combinations of electrochemical reactants well known in the art. Thus, the invention can be applied to carbon-zinc electrochemical cells, alkaline manganese dioxide cells, and lithium cells, as well as primary and rechargeable cells.
Cylindrical alkaline manganese dioxide electrochemical cells typically comprise a centrally disposed zinc anode surrounded by a plurality of ring shaped manganese dioxide cathode members. The anode and cathode are disposed within a metal container or can having an open top end. Electrical connection to the anode is generally effected by placing an elongated metal member, commonly referred to as a current collector or nail, within the zinc anode.
The nail may be forcibly driven through a resilient and electrically nonconductive seal body or gasket that forms a closure over the zinc anode material and manganese dioxide cathode material. Thus, the seal body effectively closes off the open top surfaces of the active electrochemical materials from the outside environment. The top end of the current collector protrudes above the seal body for physical and electrical connection to an electrically conductive metal bottom plate. A primary length of the elongated shank of the current collector is inserted into the zinc anode material. A small portion of the exterior surface of the shank of the current collector resides within the seal body, in sealing engagement therewith.
Prior to manufacturing a cylindrical alkaline manganese dioxide electrochemical cell, the current collector and seal body are usually preassembled to form a current collector subassembly.
The outer peripheral edge of the seal body, and the portion of the seal body surrounding the centrally located opening, are usually reinforced by thickening the seal body material at those locations. At least one portion of the seal body between the reinforced areas may be made thinner in cross-section to permit the seal body to rupture when internal pressure within the cell exceeds a predetermined limit.
The reinforced portion of the seal body that surrounds and defines the centrally located opening is commonly referred to as the "hub." When the current collector is inserted through the opening, there is preferably an interference fit, between the sidewalls defining the central opening of the hub, and the current collector because the diameter of the central opening is less than the diameter of the nail. An interference fit between the nail and the opening is required so that caustic electrolyte or other liquid chemicals in the cell cannot escape from the cell interior by creeping along the surface of the nail and through the central opening.
Depending on the particular design used in a given cell, and in addition to the two components of the current collector subassembly described above, the complete current collector assembly may additionally include a washer, a bottom plate, a gasket puncture plate a vent pressure control plate, a rivet, or any combination of the foregoing components.
Heretofore, the primary focus in designing improved current collector assemblies has been directed toward minimizing the tendency for the cells to leak electrolyte. This design goal has been addressed in numerous ways. Materials from which the seal body is fabricated have been strengthened. The current collector nails have been lubricated prior to insertion through the central opening. Tightness of the fit between the nail and the central opening, or between the outer peripheral edge of the seal body and the inner surface of the can, has been increased.
It is common for reactions taking place within an electrochemical cell to generate gases as product of such reactions. The generation of such gases causes well known increase in pressure within the cell, generally referred to as "internal pressure." Internal pressure within a cell using the current collector assembly 10 of FIG. 1(a) causes seal body 14 to deflect upwardly. The higher the pressure, the greater the deflection in the seal body.
While it is preferable to maintain all materials, including gases, sealed within the cell, in some environments, and in some individual cells, it is necessary to provide for release of internal cell pressure in preference to allowing the pressure to build up to the point where the crimp is opened up, dismembering the cell, or worse, the cell may explode or otherwise burst. Thus, in cell types which carry significant potential for bursting, the design typically provides for release of gases from the cell at a predetermined internal pressure. On the other hand, if the cell is generally open to the atmosphere, electrolyte can evaporate from the cell, causing the cell to dry out and become useless. Thus, while venting of the cell should be provided for, the cell should generally be closed to gaseous escape except when the predetermined internal pressure is reached.
In addition, the cell must be sufficiently rugged that it tolerates external impacts, especially impacts on its anode and cathode ends without deleterious effect on the integrity of the cell, or its operation. Specifically, the cell must withstand being dropped on either end without excessive denting of the cell body. In a test of particular interest, a flashlight having 2 or more cells therein is dropped on its tail end, causing the protruding cathode nubbin of the trailing cell to impact against the center of the bottom plate of the anode assembly of the cell at the lens-end of the flashlight. Desirably, the cell is sufficiently robust, structurally, that the bottom plate of the anode end is not appreciably damaged in such test. In addition, the cell should be able to withstand being dropped on the anode assembly without significant damage to the anode end, or otherwise to the cell.
The combined requirements that the cell not leak electrolyte, that the cell properly vent excess pressure within the cell, and that the cell be sufficiently robust to withstand significant external impacts, tend to compete with each other. The leak-tightness and robust structural requirements demand a strong cell structure. The requirement for venting gas demands that at least some part of that robust structure develop a gaseous leak under certain conditions.
As discussed hereinafter, cells known to the inventors tend to fall into one of two classes. In the first class, the cells are sufficiently robust that they withstand the above-described impact testing. They generally do not leak electrolyte. And they generally vent internal pressure as required. However, such cells have a relatively large number of piece parts and thus carry an undesirably high cost of production.
In the second class, the cells generally do not leak electrolyte unless they are subjected to significant external impacts at the anode end. However, upon such external impacting, the central region of the bottom plate of the cell is significantly dented, with subsequent leakage of electrolyte being common. In addition, the external impact commonly activates the venting of the cell, independent of the internal pressure, within the cell.
Thus, there remains a practical requirement for a cell having the desired combination of high strength at the anode end of the cell, tightness against leakage of electrolyte, a generally effective gaseous seal, along with proper venting, all at an economic cost, preferably using a minimum number of pieces.